Ad Astra is described as 'Open-ended space exploration game which has attempted to model a Galaxy of over 110,000 real stars based on the 1997 Hipparcos Star Catalogue. The application supports fractal planets, procedural flora and fauna with uninterrupted game play whilst within a' and is an app. There are three alternatives to Ad Astra for a variety of platforms, including Windows, Linux, Android, iPhone and iPad apps. The best Ad Astra alternative is Space Engine. It's not free, so if you're looking for a free alternative, you could try Oolite.
Space Engine - a real-time 3D space simulator, procedural universe generator and visualiser. It lets you explore the universe in three dimensions, starting from planet Earth to the most distant galaxies.
Oolite is a space sim game, inspired by Elite. Among Oolite's several similarities to its inspirational source, the gaming experience is enhanced by the context set in Elite's original manual, and the accompanying novella, The Dark Wheel.
Space Simulator is a realistic space flight simulator for iOS and Android. It is an open-ended simulation where players can free roam Solar System or reenact historical NASA space program missions.
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